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William "Billy" Karesh

Dr. William Karesh is the Executive Vice President for Health and Policy at EcoHealth Alliance.
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Dr. Jonathan Epstein Returns from Studies of Nipah Virus in Malaysia

September 14, 2005

Dr. Jonathan Epstein, Senior Research Scientist with the Consortium for Conservation Medicine, based at Wildlife Trust, recently returned from Malaysia, where he works closely with Malaysian, American and Australian Scientists on a large international, collaborative study of the ecology of Nipah virus in Malaysia, funded by the National Institutes of Health.

Dr. Epstein's field work focuses on the epidemiology of Nipah virus in its natural wildlife reservoir, giant fruit bats, or "flying foxes," of the genus Pteropus . His work takes him throughout Peninsular Malaysia in search of flying foxes, which are declining in numbers due to hunting and deforestation.

There are two species of flying fox native to Malaysia: the Malayan flying fox ( Pteropus vampyrus ) and the Variable flying fox ( Pteropus hypomelanus ). Both are known to carry Nipah virus ( P. vampyrus is the largest bat species in the world, with a wingspan of over 4 feet.) These magnificent species roost in trees by the hundreds or thousands and feed on fruit and nectar. They are critical seed dispersers and pollinators, making them very important for rainforest propagation.

Dr. Epstein works closely with colleagues from the Veterinary Research Institute in Ipoh, Malaysia, to monitor flying fox populations all over the country, and test them for Nipah virus in order to better understand to what degree these populations are infected. This is part of a larger study that is testing several hypotheses on why Nipah virus emerged in 1998, and whether it is likely to happen again.

In the past two months, Dr. Epstein focused his field work in two locations: Lenggong, Perak, which is in northern Malaysia and close to the pig farm where Nipah virus first emerged, and Benut, Johor, on the Southwest coast on the Straits of Malacca, close to neighboring Sumatra. Dr. Epstein's work with satellite telemetry has shown that flying foxes are capable of flying large distances, and that they travel between Sumatra and Malaysia. Dr. Epstein has just placed two more satellite collars on Malayan flying foxes roosting in Benut.

In this WT online interview, Dr. Epstein articulates the disease and its global ramifications, disease transmission among species, and the key questions of his research.


1. What is the Nipah virus and why is it viewed as such a global health threat?

Nipah virus is a paramyxovirus, within the same family of viruses as measles. It causes fever and encephalitis in humans, and can carry between a 40% and 70% case fatality rate, making it among the most lethal emerging infectious diseases known.

Nipah virus emerged in Malaysia, as a pig disease. It had devastating agricultural and economic consequences, as over a million pigs were culled to stop the outbreak, hundreds of farms closed and thousands of jobs lost - all costing the Malaysian economy over $450 million USD. Nipah virus spread from pigs to humans, killing 40% of those infected, making it an important public health issue as well.

Nipah virus spread readily among pigs, especially in the index farm, which was a high-production facility (over 30,000 pigs). Pigs acted as an amplifier, aerosolizing virus and infecting humans. Nipah virus could potentially be used as a destructive biological weapon for either agro- or bioterrorism. The outbreaks in Bangladesh showed some evidence that human-to-human transmission was possible, meaning that Nipah virus could potentially create a wide-spread pandemic, given the opportunity.

2. Nipah is a zoonotic disease, a disease that passes from animals into human populations. In Malaysia, bats are considered the prime carrier. What has caused the emergence of Nipah from bats to humans?

In Malaysia, the cause was multifactoral; the virus has likely been in bats for a long time. Our experience with Nipah virus and Hendra virus ( a related virus carried by Australian fruit bats) so far is that a very low percentage of bats seem to be actively infected at any one time. The expansion of the pig farming industry brought a large number of susceptible animals (pigs) into closer contact with wildlife (bats) by being constructed next to rainforest habitat. The increased contact between bats and pigs created more opportunities for transmission to occur.

The pig farm on which Nipah emerged also had large fruit orchards, situated next to the pig enclosures. Fruit bats have long utilized cultivated fruit as a food resource, so this was a natural attractant for bats, which roost in primary rainforest near the pig farms, and forage within 30 miles of their roosting site.

The size of the pig farm on which Nipah emerged, specifically the density of pigs kept on the farm and the high rate of import of piglets and young pigs (weaners and porkers) to be raised and sold, kept a steady stream of susceptible individuals available to sustain a Nipah outbreak. Nipah virus, like measles, depends on a high density of susceptible individuals (e.g. measles outbreak in a school, or city) to maintain an outbreak. The original outbreak farm exceeded the minimum density of pigs necessary to sustain an outbreak, thus when Nipah spilled over from bats to pigs, the outbreak was maintained long enough to infect many pigs and then humans, which is how it was noticed.

The fruit trees were seen to be growing over the pig farms, and so we hypothesize that bats infected with Nipah dropped excretia (saliva via fruit solids; urine, or feces) into the pig pens, thus infecting the pigs. We also hypothesize that hunting and loss of habitat influence bat movements. It may be that as bats continue to lose natural food resources through land-clearing activities, they will increasingly depend on orchards.

3. What are the key questions for your investigation?

  1. Did the density of pigs on the index farm play a significant role in the initial outbreak of Nipah?
  2. How do flying foxes (fruit bats of the genus Pteropus) in Malaysia maintain Nipah virus infection within their own populations?
  3. Has hunting and deforestation altered the long-range movements of flying foxes in Malaysia?

4. What is your process for completing the research?

We have built an international, multi-disciplinary collaboration between CCM, the Malaysian government, and the Australian government to study Nipah virus and its counterpart virus, Hendra virus (in Australia). We use hypothesis-driven research to answer the following questions about Nipah virus in Malaysia.

  • We are doing a cross-sectional survey of the two species of pteropid bat native to Peninsular Malaysia (The Malayan flying fox (Pteropus vampyrus) and the Variable flying fox (Pteropus hypomelanus) to understand the distribution of this virus across bat populations.
  • We are conducting a longitudinal study of both bat species to see how antibody titers change over time within a population.
  • We are using parameterized mathematical modeling to determine whether pig density on farms was an essential variable for the outbreak to occur, using historical data from the original outbreak in Malaysia to compare the characteristics of infected and uninfected pig farms
  • We are using state-of-the-art satellite telemetry to understand the long-range movements of the natural reservoir of Nipah virus, the Malayan flying fox (Pteropus vampyrus).
  • We are using laboratory studies to develop more sophisticated diagnostic techniques and to better understand the pathophysiology of Nipah virus in pteropid bats.
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